Here's a not very helpful answer from me. I went to a very selective school, and we were setted. The top set for maths was taught in a very different style from the bottom set for maths although we all sat the same o'level and there would still have been some A grades coming out of the bottom set. In the top set we had to find out more for ourself, less spoonfeeding, more coverage of stuff that was not needed for the exam (I don't mean A' level stuff). In some classes I remember they were flexible enough to let the people who were really able in that subject do something completely different e.g. I remember someone who was great at Latin working their way through completely different material from the rest of us quite a bit of the time. And how about this ........ they let us read under the desk so long as we were doing well ....... they didn't tell us it was OK, but they never told anyone not to unless they clearly needed to do better!!! So we read surreptitiously when we knew we could get by with half an ear on what was going on, and the teachers pretended they hadn't noticed.